Peripatus is zoologically very interesting, because it exhibits both arthropod and annelid characters as under. Peripatus is supposed to be the connecting link between Annelida and Arthropoda.

In this study, we propose a new biometric methodology for annelids, based on the number of chaetiger segments as an indicator of growth (independent variable), relating to other structures, such as the number of paragnaths and parapodia, using the commercial species Hediste diversicolor (Müller, 1776) as a model. This species belongs to the family Nereididae, found in estuarine environments along the European Atlantic coast (DOI 10.11646/zootaxa.5696.1.2). This species has several appendages with sensory functions along the body that help the animal navigate in its environment. It also has specialised feeding structures in the peristomium that are essential for taxonomic identification, called paragnaths. The parapodium has distinct chaetal arrangements that distinguish it from other species in the same genus. Due to the importance of economic activities such as fishing and aquaculture, this ragworm is a typical protagonist for studies in multiple areas, with a well-documented life cycle. However, knowledge is lacking on the growth and evolution of individual structures and appendages of the body in this species during its life cycle. Our findings revealed a significant positive correlation between the number of chaetiger segments and most of the morphological structures analysed, evidencing a proportional growth of most of these features, related directly to the chaetigers segments. Simple chaetae, a specific type of chaeta only present in the genus Hediste, were only found in individuals with more than 25 chaetiger segments, possibly indicating a functional change as the worm matures. Simple chaetae tend to be found earlier along the body axis, closer to the median chaetiger as the organism matures. The results illustrate how H. diversicolor develops over time, highlighting the developmental processes and representing the first biometric study of nereidid polychaetes, based on chaetiger count as a proxy for biometric growth, to define new possible life cycle stages, for supporting aquaculture purposes and other research fields.